Voltage measurement circuit with AC-coupled diferential amplifier

ABSTRACT

A voltage measurement circuit is provided, which is has an AC-coupled differential amplifier coupled to first and second input terminals for measuring a voltage across the first and second input terminals. The AC-coupled differential amplifier blocks a DC component of the voltage and presenting a low input bias current to the first and second input terminals while isolating the first and second input terminals from charge stored in capacitances in the AC-coupled differential amplifier.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of U.S. application No.09/784,782, filed Feb. 15, 2001 and entitled “A TRASFER CURVE TESTER FORTESTING MAGNETIC RECORDING HEADS”, which claims priority from U.S.Provisional Application No. 60/182,826, filed Feb. 16, 2000, andentitled “HIGH PERFORMANCE TRANSFER CURVE TESTER AND TEMPERATURECOMPENSATED HALL SENSOR.”

FIELD OF THE INVENTION

[0002] The present invention relates to data storage systems and, moreparticularly, to a transfer curve tester having a bi-directional currentsource for testing magnetic recording heads used in data storagesystems.

BACKGROUND OF THE INVENTION

[0003] Many data storage systems use magnetic or magneto-opticalrecording heads for writing information to and reading information froma magnetic medium. For example, disc drives of the “Winchester” typehave one or more rigid discs, which are coated with a magnetizablemedium for storing digital information in a plurality of circular,concentric data tracks. The discs are mounted on a spindle motor, whichcauses the discs to spin and the surfaces of the discs to pass underrespective head suspension assemblies. Head suspension assemblies carrytransducers which write information to and read information from thedisc surface. An actuator mechanism moves the head suspension assembliesfrom track-to-track across the surfaces of the discs under control ofelectronic circuitry. “Floppy-type” disc drives use flexible discs,which also have circular, concentric data tracks. For a tape drive, theinformation is stored along linear tracks on the tape surface.

[0004] In these applications, several different types of transducershave been used that rely on magnetic properties for writing to and/orreading from the magnetic medium. For an inductive-type transducer, thedirection of current through the transducer is controlled during a writeoperation to encode magnetic flux reversals on the surface of the mediumwithin the selected data track. When retrieving data from the medium,the inductive transducer is positioned over the data track to sense theflux reversals stored in the data track and generate a read signal basedon those flux reversals. In a magnetoresistive type of transducing head,the flux reversals cause a change in the resistance of the head, whichis sensed by a detector circuit. Typically, a reference current ispassed through the magneto-resistive head and the change in resistanceis sensed by measuring changes in the voltage across the head. Othertypes of detecting circuits can also be used.

[0005] In order to understand the basic physics of a magnetictransducing head during development and manufacturing, it is common totest the response of the head to an applied magnetic field. For example,one series of tests is known as “Transfer Curve Testing”. To generate atransfer curve for a particular transducing head, the head is placed ina magnetic field (steady state or time varying) and the output signalfrom the transducing head is measured. The transfer curve is simply aplot of the output signal versus the applied magnetic field, where thefield is varied from some negative value to some positive value, whichis usually the same magnitude as the negative value. For amagneto-resistive type of head, the output signal consists of a steadystate voltage, which is a function of the bias current applied to thehead, the bulk resistance of the head and the applied magnetic field.Typical characteristics that can be measured from the transfer curvedata include read signal amplitude at maximum field, noise with zerofield, noise with applied field, linearity over some range of field, andsymmetry. Symmetry is a comparison of the read signal amplitude with amaximum positive field and the read signal amplitude with a maximumnegative field.

[0006] The rapidly changing technology in magnetic recording heads hascreated a wide range of operating requirements for the heads as well asa wide range of head performances. For example, reference bias currentrequirements for a transfer curve tester can vary from tens ofmicro-Amperes to many tens of milli-Amperes, and the transfer curvetester may require tens of volts to drive the reference current. Formagneto-resistive types of heads, the amplitudes of output voltages thatmust be measured can range from tens of micro-volts to tens ofmilivolts, while the resistance of the head can range from tens of Ohmsto hundreds of Ohms. Also, the steady-state voltage output due to thereference bias current is typically hundreds of millivolts, but can beas large as tens of volts with special devices.

[0007] These wide ranges of operating requirements and head performancesset very challenging requirements for the measurement electronics. Forexample, in order to measure the noise of a 50 Ohm head, the noiseintroduced by the transfer curve tester should be less than 1 nV/{squareroot}Hz. Bias currents of 10 micro-Amperes require a current source withan accuracy of better than 100 nAmps, and the input bias currents drawnby the measurement electronics should be similar to prevent measurementerrors. All of these requirements, when coupled with a potentially largeDC bias voltage, present a difficult design challenge for themeasurement electronics in the transfer curve tester. Typically one ormore of these requirements is substantially compromised.

[0008] Thus, a transfer curve tester having improved measurementelectronics and an accurate current source is desired.

SUMMARY OF THE INVENTION

[0009] One embodiment of the present invention is directed to a voltagemeasurement circuit, which is has an AC-coupled differential amplifiercoupled to first and second input terminals for measuring a voltageacross the first and second input terminals. The AC-coupled differentialamplifier blocks a DC component of the voltage and presenting a lowinput bias current to the first and second input terminals whileisolating the first and second input terminals from charge stored incapacitances in the AC-coupled differential amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view a typical head suspension assemblyfor a rigid disc drive.

[0011]FIG. 2 is a simplified diagram of a head testing apparatusaccording to one embodiment of the present invention.

[0012]FIG. 3 is a diagram of a head transfer curve tester having abalanced, bi-directional current source and a low-noise measurementcircuit according to one embodiment of the present invention.

[0013]FIG. 4 is a diagram of an input stage used in the tester shown inFIG. 3, according to one embodiment of the present invention.

[0014]FIG. 5 is a diagram of a differential amplifier stage used in thetester shown in FIG. 3, according to one embodiment of the presentinvention.

[0015]FIG. 6 is a diagram of a differential amplifier stage according toan alternative embodiment of the present invention.

[0016]FIG. 7 is a diagram of the balanced, bidirectional current sourceshown in FIG. 3, according to one alternative embodiment of the presentinvention.

[0017]FIG. 8 is a diagram of a current control circuit used in thecurrent source shown in FIG. 7, according to one alternative embodimentof the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0018]FIG. 1 is a perspective view of a typical head suspension assemblyfor a rigid disc drive. Head suspension assembly 100 includes suspension102, flexure 104 and slider 106. Slider 106 carriers a transducer or“head” for writing to and/or reading from a disc surface in a discdrive. Slider 106 can carry a variety of different types of transducersin alternative embodiments, such as an inductive-type transducer, amagneto-resistive type transducer, a giant magneto-resistive transducer,a spin tunnel junction transducer or a magneto-optical transducer.During operation, suspension 100 is attached to an actuator mechanism(not shown), which moves suspension 100 and the transducer carried byslider 106 from track-to-track across the surface of the disc undercontrol of electronic circuitry.

[0019] In order to understand the basic physics of a magnetic recordinghead during development and manufacturing, it is common to test theresponse of the head to an applied magnetic field, prior to assemblingthe head in a drive. For example, one series of tests is known as“Transfer Curve Testing”. To generate a transfer curve for a particularrecording head, the head is placed in a magnetic field and the outputsignal from the head is measured. The transfer curve is simply a plot ofthe output signal versus the applied magnetic field, where the field isvaried from some negative value to some positive value, which is usuallythe same magnitude as the negative value.

[0020]FIG. 2 is a simplified diagram of an apparatus for applying amagnetic field to a recording head during transfer curve testing,according to one embodiment of the present invention. The particulartesting apparatus shown in FIG. 2 is provided as an example only. Itshould be understood that any suitable apparatus can be used to generatea magnetic field for testing a magnetic head or other magneticallyresponsive device in accordance with alternative embodiments of thepresent invention. Magnetic field generating apparatus 200 includes base202, magnetic cores 204 and 206, windings 208 and 210 and air gap 212.Cores 204 and 206 are arranged to generate a magnetic field (simple orcomplex) across air gap 212 in orthogonal directions when excited bywindings 208 and 210. Air gap 212 forms a test volume for receiving amagnetic transducer under test. The magnetic transducer is inserted intoair gap 212 along axis 214 by a suitable positioning device. In oneembodiment, air gap 212 is sized to receive the distal end of a headsuspension assembly, such as that shown in FIG. 1, so as to positionslider 106 and is attached transducer between opposing faces of cores204 and 206. However, air gap 212 can be sized to receive a plurality ofhead suspension assemblies, such as those carried by an E-block actuatorassembly in alternative embodiments.

[0021] When the head being tested is positioned within air gap 212,windings 208 and 210 are excited to generate a selected magnetic fieldin air gap 212 according to a predetermined test pattern. The responseof the transducer being tested is then measured and analyzed. Hallsensors 220 and 222 are positioned relative to air gap 212 to measurethe strength of the magnetic field that is applied across the air gap.In one embodiment, Hall sensors 220 and 222 are supported by amagnetically permeable material 224, which is positioned about air gap212 between cores 204 and 206. Hall sensor 220 measures the magneticfield generated between the opposing faces of core 204, and Hall sensor222 measures the magnetic field generated between the opposing faces ofcore 206.

[0022]FIG. 3 is a diagram of a circuit 300 for electrically biasing therecording head and measuring its response to the magnetic field appliedby the apparatus shown in FIG. 2, according to one embodiment of thepresent invention. Circuit 300 includes input terminals 301 and 302 forcoupling to a recording head 303 under test. In one embodiment, inputterminals 301 and 302 include electrical sockets that are configured forquickly exchanging one recording head 303 for another within circuit 300as the transfer curve or other characteristics of each recording head ismeasured.

[0023] Circuit 300 further includes a current source 304, which iscoupled in parallel with input terminals 301 and 302 for applying areference bias current, I_(HEAD), to recording head 303. In oneembodiment, current source 304 is a balanced, bi-directional currentsource that is capable of supplying large ranges of currents at lownoise. A measurement circuit 305 is coupled to input terminals 301 and302 for measuring the response of head 303, as biased by I_(HEAD) andexcited by the magnetic field applied by magnetic field generatingapparatus 200 (shown in FIG. 2). Measurement circuit 305 has ameasurement output 306.

[0024] In one embodiment, a control circuit 310 controls the sequence oftests performed on each recording head 303 through control outputs 311and 312 while measuring the resulting head response on input 313.Control output 311 is coupled to a current control input 314 of currentsource 304 for controlling the level of bias current, I_(HEAD), appliedto recording head 303 through input terminals 301 and 302. The controlsignal applied to current control input 314 can include a referencevoltage or current, for example. In an alternative embodiment, currentsource 304 is a fixed, not adjustable current source. Output 311 iscoupled to magnetic field generating apparatus 200 (shown in FIG. 2) forcontrolling the magnetic field applied to recording head 303. Input 313is coupled to measurement output 306 for receiving a measure of the headresponse for each level of applied magnetic field.

[0025] For each level of applied magnetic field, control circuit 310stores the measured head response in memory 316 or supplies the measuredhead response to user interface 318. The stored response data, whenplotted as a function of magnetic field, forms a transfer curve for therespective recording head 303 under test as the magnetic field is variedfrom some negative value to some positive value. Typical characteristicsthat can be measured from the stored transfer curve data include readsignal amplitude at maximum field, noise with zero field, noise withapplied field, linearity over some range of field, and symmetry.Symmetry is a comparison of the read signal amplitude with a maximumpositive field and the read signal amplitude with a maximum negativefield. Control circuit 310 can be implemented with a discrete analogcontrol circuit, a discrete digital control circuit or state machine, ora programmed computer such as a microcontroller, for example. Othertypes of control circuits can also be used.

[0026] For a magneto-resistive type of head, the head response consistsof a steady state voltage on tester inputs 301 and 302, which is afunction of the reference bias current I_(HEAD) applied to recordinghead 303, the bulk resistance of the head and the magnetic field appliedby the apparatus shown in FIG. 2. As described in more detail below,measurement circuit 305 quickly measures changes in the voltagedeveloped across recording head 303 while drawing a very low input biascurrent, introducing a very low noise level into the measurement, andisolating the recording head from potentially harmful capacitanceswithin the measurement circuit.

[0027] Measurement circuit 300 includes input stages 320 and 322,AC-coupling capacitors 324 and 326, analog switches 328 and 330, biasresistors 332 and 334 and differential amplifier 336. Input stages 320and 322 have inputs coupled to input terminals 301 and 302,respectively, and outputs coupled to capacitors 324 and 326,respectively. Each input stage 320 and 322 includes at least onebuffering amplifier, such as an operational amplifier coupled as avoltage follower. In one embodiment, the buffering amplifiers have unitygain, but can apply other gain factors in alternative embodiments. Avariety of common operational amplifiers can be used, such as LT1128operational amplifiers available from Linear Technology Corporation,which draw input bias currents on the nano-Ampere level.

[0028] The use of buffering amplifiers in input stages 320 and 322therefore provides low input bias currents to input terminals 301 and302, which are significantly lower than the current that is used to biasrecording head 303. This allows for greater accuracy in the measuredvoltage across terminals 301 and 302. Input stages 320 and 322 alsoprotect head 303 from any stray charges that might be present oncapacitors 324 and 326 as successive heads 303 are inserted and removedfrom the test socket. As recording heads continue to become smaller,these heads can be damaged more easily by such stray charges. Anotherfunction of input stages 320 and 322 is to isolate input terminals 301and 302 from differential amplifier 336 so that the amplifier will notalter the operating point of the recording head 303 under test. Inaddition, input stages 320 and 322 provide a strong drive current toquickly charge and discharge capacitors 324 and 326 and thus the inputsto differential amplifier 336 when the DC operating point of head 303 ischanged. This quickens the settling time at measurement output 306. Evenhigher drive and lower noise can be achieved by coupling multipleoperational amplifiers in parallel with one another within each inputstage 320 and 322.

[0029] Capacitors 324 and 326 are coupled between input stages 320 and322 and inputs 340 and 342 of differential amplifier 336 to block any DCvoltage component of the head response from reaching the amplifierinputs. Bias resistors 332 and 334 are coupled between respectiveamplifier input terminals 340 and 342 and ground terminal GND forbiasing each input of amplifier 336. Differential amplifier 336 has anamplifier output 348, which is coupled to measurement output 306 ofmeasurement circuit 305. Capacitors 324 and 326 and bias resistors 332and 334 together form a high-pass filter. The capacitances of ACcoupling capacitors 324 and 326 and the resistances of resistors 332 and334 are selected to achieve a desired high-pass corner frequency for thefilter as well as a desired noise level introduced by the filter. Thehigh-pass corner frequency is determined by 1/(2πRC), where R is theresistance of resistors 332 and 334 and C is the capacitance of ACcoupling capacitors 324 and 326. In a typical transfer curve tester,this corner frequency is often less than 1 Hz. In order to prevent noiseintroduced by biasing resistors 332 and 334 from affecting the compositenoise of the filter, resistors 332 and 334 are made as small as possibleand the AC coupling capacitors 324 and 326 are made as large aspossible. With the drive that is available from input stages 320 and322, resistances of less than 100 ohms can easily be used. The large ACcoupling capacitors provide a low impedance path through input stages320 and 322 to shunt the resistor noise to ground.

[0030] Differential amplifier 336 can include any discrete or commercialdifferential amplifier that meets the desired noise and bandwidthrequirements. For example, a suitable commercial instrumentationamplifier having low noise (1 nV/{square root}Hz) and high gain is theSSM2017 amplifier, which is available from Analog Devices. For low noiseat lower gains, an instrumentation amplifier made from multipleoperational amplifiers such as the LT1128 or AD797 can be used. TheAD797 is also available from Analog Devices. For extremely low noise(less than 0.5 nV/{square root}Hz), multiple operational orinstrumentation amplifiers can be coupled together in parallel with oneanother.

[0031] The settling time of amplifier 336 to a change in the input DClevels at input terminals 301 and 302 is determined by the RC timeconstant of the filter formed by AC coupling capacitors 324 and 326 andbias resistors 332 and 334. This settling time can be very long, whichcan impact the time required to fully test each head 303 if there are alarge number of measurement levels for each head. To circumvent thisproblem, analog switches 328 and 330 are coupled in parallel across biasresistors 332 and 334, respectively. Switches 328 and 330 have switchcontrol inputs 344 and 346, which are coupled to and controlled bycontrol circuit 310. When control circuit 310 changes the head biascurrent I_(HEAD) through current control input 314, control circuit 310closes switches 328 and 330 through switch control inputs 344 and 346for a time sufficient for capacitors 324 and 326 to fully charge. Whenswitches 328 and 330 are closed, the RC time constant is reduced byseveral orders of magnitude, allowing the differential amplifier 336 tosettle quickly to the new DC operating point. Control circuit 310 thenopens switches 328 and 330 and records the response on measurementoutput 306 in memory 316.

[0032]FIG. 4 is a diagram illustrating one of the input stages 320 and322 in greater detail. Each input stage 320 and 322 includes an input400, an output 402 and a plurality of buffer amplifiers 404, which arecoupled in parallel with one another. Each buffer amplifier 404 iscoupled to operate as a voltage follower. The output of each amplifier404 is coupled to buffer output 402 through a respective summingresistor 406. Input stages 320 and 322 can also be designed to providegain from buffer input 400 to buffer output 402 or can be simple bufferswith unity gain as shown in FIG. 4. By coupling a plurality ofamplifiers 404 in parallel with one another, an input stage having a lownoise level, such as less than 0.5 nV/{square root}Hz, an input biascurrent in the nano-amp range and a moderately large drive current canbe realized.

[0033] The input bias current drawn through buffer input 400 and thecurrent driven through buffer output 402 go up in proportion to thenumber of devices used, i.e.:

[0034] Total I_(BIAS)=N*I_(BIAS) of each device; and

[0035] Total I_(OUT)=N*I_(OUT) of each device

[0036] where N is the number of parallel-connected amplifiers in eachstage.

[0037] Conversely, the noise of input stages 320 and 322 go down by thesquare root of the number of devices used, i.e.:

V _(NOISE TOTAL) =[V _(NOISE OF EACH DEVICE) ]/{square root}N

[0038]FIG. 5 is a diagram illustrating differential amplifier 336 ingreater detail according to one embodiment of the present invention.Amplifier 336 includes a plurality of individual amplifiers 500 coupledin parallel with one another. Summing resistors 502 are coupled inseries between the outputs of respective amplifiers 500 and differentialamplifier output 348. Again, the use of multiple operational orinstrumentation amplifiers in parallel with one another can provide forextremely low noise (less than 0.5 nV/{square root}Hz) withindifferential amplifier 336.

[0039]FIG. 6 is a diagram illustrating differential amplifier 336 ingreater detail according to an alternative embodiment of the presentinvention, which uses multiple operational amplifiers 600 for providinglow noise at a lower gain. The same reference numerals are used in FIG.6 as were used in FIG. 3 for the same or similar elements. Other typesof differential amplifier circuits can also be used in accordance withthe present invention.

[0040]FIG. 7 is a block diagram which illustrates current source 304 ingreater detail according to one embodiment of the present invention.Current source 304 is a balance, bidirectional current source thatincludes current control circuit 700, amplifiers 702 and 704, senseimpedances 706 and 708, analog voltage inverter 710 and current outputterminals 712 and 714. Current control circuit 700 includes currentcontrol input 314, feedback inputs 720 and 722 and control output 724.Current control circuit 700 generates a control voltage on controloutput 724 based on the voltage received on current control input 314from control circuit 303 (shown in FIG. 3) and a voltage developedacross feedback inputs 720 and 722. The control voltage on controloutput 724 is coupled to the input of amplifier 702 and to the input ofanalog voltage inverter 710. Analog voltage inverter 710 inverts thecontrol voltage on control output 724 and provides the inverted controlvoltage to the input of amplifier 704. In an alternative embodiment,analog voltage inverter 710 is removed, and amplifier 704 is replacedwith an inverting amplifier.

[0041] Amplifiers 702 and 704 can include operational amplifiers orclass A/B bi-polar amplifiers, for example. Other types of amplifierscan also be used. In one embodiment, amplifiers 702 and 704 have unitygain, but can have other gain values in alternative embodiments. Toprovide a balanced, bidirectional current to current output terminals712 and 714, amplifiers 702 and 704 are matched to one another, witheach amplifier having the same input bias current, the same output drivecurrent and the same gain from input to output.

[0042] The outputs of amplifiers 702 and 704 are coupled to senseimpedances 706 and 708. Sense impedance 706 is coupled in series betweenthe output of amplifier 702 and current output terminal 712. Similarly,sense impedance 708 is coupled between the output of amplifier 704 andcurrent output terminal 714. In one embodiment, sense impedances 706 and708 each include a resistance coupled in series between the respectiveamplifier output and the respective current output terminal 712 and 714.Sense impedances 706 and 708 are matched to one another to provide abalanced differential output current through current output terminals712 and 714.

[0043] The voltage developed across sense impedance 706 is fed back tofeedback inputs 720 and 722 of current control circuit 700. Currentcontrol circuit 700 measures the voltage developed across the senseimpedance at feedback inputs 720 and 722 and compares this voltage tothe reference voltage provided on current control input 314. Based onthis comparison, current control circuit 700 adjusts control output 724such that the desired current level is supplied through current outputterminals 712 and 714, as represented by the voltage drop across senseimpedance 706.

[0044]FIG. 8 is a schematic diagram, which illustrates current controlcircuit 700 in greater detail, according to one embodiment of thepresent invention. Current control circuit 700 includes buffer andsignal conditioning circuit 800, summation node 802, integrator 804,which is formed by amplifier 806 and capacitor 808, and differentialamplifier 810. The reference voltage provided on current control input314 is coupled to an addend input 812 of summation node 802. Feedbackinputs 720 and 722 are coupled to respective inputs of differentialamplifier 810. The output of differential amplifier 810 is coupled to asubtrahend input 814 of summation node 802. The output 816 of summationnode 802 is coupled to the non-inverting input of amplifier 806 withinintegrator 804. The inverting input of amplifier 806 is coupled toground terminal GND. Capacitor 808 is coupled between the output and thenon-inverting input of amplifier 806. The output of amplifier 806 formscontrol output 724 of current control circuit 700. In operation,summation node 802 compares the reference voltage received on currentcontrol input 314 to the voltage measured across sense impedance 706(shown in FIG. 7) and provides the difference to integrator 804. Inresponse, integrator 806 adjusts the voltage on control output 724.

[0045] Current source 304 provides a balanced, differential outputcurrent with a large voltage output using off-the-shelf operationalamplifiers. A balanced differential output, while doubling the availablecurrent for driving high-resistance recording heads, also assures thatthe measurement system has low noise by providing true “4-point”measurement capability.

[0046] It is to be understood that even though numerous characteristicsand advantages of various embodiments of the invention have been setforth in the foregoing description, together with details of thestructure and function of various embodiments of the invention, thisdisclosure is illustrative only, and changes may be made in detail,especially in matters of structure and arrangement of parts within theprinciples of the present invention to the full extent indicated by thebroad general meaning of the terms in which the appended claims areexpressed. For example, the current source and measurement circuit canbe used together or independently in applications other than testingrecording heads. The current source can be used in any application inwhich a bi-directional current source is useful. The measurement circuitcan be used in any application requiring or benefiting from a low inputbias current, low noise and high accuracy. Also, individual componentscan be implemented with analog circuit elements, digital circuitelements or a combination of both.

What is claimed is:
 1. A voltage measurement circuit comprising: firstand second input terminals; and voltage measurement means coupled to thefirst and second input terminals for measuring a voltage across thefirst and second input terminals with an AC-coupled differentialamplifier, which blocks a DC component of the voltage, and forpresenting a low input bias current to the first and second inputterminals while isolating the first and second input terminals fromcharge stored in capacitances in the AC-coupled differential amplifier.2. The voltage measurement apparatus of claim 1 wherein the voltagemeasurement means comprises: a first input stage comprising a firstbuffer input, which is coupled to the first input terminal, a firstbuffer output and a plurality of parallel-connected buffer amplifierscoupled between the first buffer input and the first buffer output; asecond input stage comprising a second buffer input, which is coupled tothe second input terminal, a second buffer output and a plurality ofparallel-connected buffer amplifiers coupled between the second bufferinput and the second buffer output; and the AC-coupled differentialamplifier, which has first and second amplifier inputs coupled to thefirst and second buffer outputs through first and second capacitors,respectively, and having a measurement output.
 3. The voltagemeasurement apparatus of claim 2 wherein each buffer amplifier in thefirst and second input stages comprises an operational amplifier coupledto operate as a voltage follower.
 4. The voltage measurement apparatusof claim 3 wherein the first and second input stages have unity gain. 5.The voltage measurement apparatus of claim 1 wherein the differentialamplifier comprises an instrumentation amplifier.
 6. The voltagemeasurement apparatus of claim 2 wherein the differential amplifiercomprises a plurality of individual differential amplifiers coupled inparallel to one another between the first and second amplifier inputsand the measurement output.
 7. The voltage measurement apparatus ofclaim 2 wherein the voltage measurement means further comprises: firstand second amplifier biasing resistors coupled between a voltagereference terminal and the first and second amplifier inputs,respectively; and first and second switches coupled in parallel with thefirst and second amplifier biasing resistors, respectively, and eachhaving a control input for selectively shorting the respective first andsecond amplifier bias resistors.
 8. The voltage measurement apparatus ofclaim 1 wherein the voltage measurement means further comprises abalanced, differential current source coupled to the first and secondtester input terminals.